A particular application of the invention lies in warning a person when that person becomes separated by more than a certain distance from an article, another person, or an animal for which said person is the holder, guardian, or minder. For example, the article can be an item of personal belongings, of baggage, a key, or a bunch of keys, and the purpose of the apparatus of the invention is to warn its holder that the article has been forgotten or that the holder is too far away to oversee it properly.
Another application of the invention lies in warning a person when that person or some article, other person, or animal approaches a particular place closer than a certain distance, e.g. corresponding to a safety perimeter.
French patent application FR 2 676 135 describes a system that is intended more particularly for indicating that a child has gone further than a predetermined distance from its minder. A transmitter unit carried by the child sends a signal to a fixed station near the minder. The fixed station comprises a receiver associated with a circuit for detecting the level of the signal from the transmitter. When received signal level drops below a predetermined threshold because the separation distance has exceeded a certain limit, then an alarm is activated at the receiver. Each of the housings containing a transmitter or a receiver needs to be provided with its own manual on/off control.
Another system intended for child-minding purposes is described in international patent application WO 93/19437. Compared with the preceding system, it presents the advantage of the transmitter and the receiver being activated and deactivated automatically when they are physically separated from each other and when they are united with each other.
International patent application WO 95/02874 relates to a system for monitoring one or more articles by means of a receiver and one or more transmitters that can be associated with respective articles to be monitored. It is stated that not only is an alarm raised when a transmitter goes beyond a certain distance from the receiver, but also that the distance and the direction of the transmitter can be measured; nevertheless no detailed description is given of means for implementing that effect.
European patent application EP 0 838 907 describes in more detail a system for raising an alarm when a mobile telephone becomes separated from a user""s belt support or holder by more than some given distance. Transceiver units are associated with the telephone and with the belt support for transmitting signals containing respective identity information and for verifying that the signals are received at satisfactory level and contain the expected identity information. The unit associated with the telephone makes use of the telephone""s resources. As soon as the telephone moves away beyond the predetermined distance, or as soon as the unit associated with the telephone no longer recognizes the identity information of the associated belt support, and vice versa, then an alarm is raised by the support and by the telephone, and the telephone switches off automatically.
International patent application WO 93/25983 describes a system comprising two ultrasound transceiver units. In one mode of operation, an alarm is raised at a first unit (a monitoring unit) when the second unit (a monitored unit) no longer receives the signal and alerts the first unit by transmitting a warning signal at high amplitude. In another mode of operation, the monitoring unit sends a signal to the monitored unit which then relays the signal back to the monitoring unit so as to be able to perform a distance measurement and modulate the intensity of the alarm accordingly, such that when the units move closer together again the sound level of the alarm increases. In the second mode of operation, the user can thus locate the monitored unit. That system requires ultrasound transmission. It is not suitable for reliable use in all environments. In addition, in the first embodiment, an alarm is raised only after a powerful warning signal has been transmitted by the monitored unit to the monitoring unit and said warning signal has been received properly by the monitoring unit.
Compared with the state of the art outlined above, an object of the invention is to propose self-contained apparatus suitable for multiple uses and capable of operating reliably and with a high degree of security in an environment containing electromagnetic interference and in the presence of other similar apparatuses operating nearby.
To this end, the invention provides apparatus for raising an alarm when a separation distance between a monitored, first unit and an alarm, second unit exceeds a threshold, in which apparatus: the first unit includes means for wireless transmission of a signal containing identity information; the second unit includes means for receiving said signal and for causing alarm means to operate when the level of the received signal is below a predetermined threshold or when the identity information is not recognized in the received signal; and means are provided to cause the first unit and the second unit automatically to take up an inactive state and an active state respectively when they are physically connected to each other and when they are physically separate from each other, in which apparatus, according to the invention:
the first unit is a self-contained unit which includes encoding means for controlling the transmission of a signal in the form of periodic frames containing identity information encoded in a form that varies with each frame; and
the second unit is a self-contained unit which includes decoding means suitable for recognizing the identity information in each received frame to cause the alarm means to operate over a predetermined time interval in response to failure to correctly receive or to recognize the identity information in a frame or in a predetermined number of consecutive frames.
Advantageously, means are provided to cause the second unit to take up its inactive state at the end of said predetermined time interval if, during said time interval, the identity information has not been recognized in at least one received frame, if any, and to interrupt operation of said alarm means if, during said predetermined time interval, the identity information is recognized in a received frame.
Transmitting identity information encoded in a form that varies with each frame provides a high level of security not only against attempts at fraud, but also against electromagnetic transmissions in the form of interference or coming from similar apparatuses operating nearby.
The fact that the receiver passes to an inactive state only after a predetermined time interval has elapsed after the alarm has been raised gives the user time to get back close to a monitored article which might have been left behind inadvertently, without deactivating the receiver. The receiver thus remains active without the user needing to perform any maneuver of any kind after the alarm has been raised.
In a variant embodiment, the second unit, or alarm unit includes a transmitter for wireless transmission of an interrogation signal, and the first unit, or monitored unit, includes a receiver for causing at least one frame to be transmitted automatically in response to receiving the interrogation signal.
This disposition makes it possible to associate a plurality of monitored units with the same alarm unit, with the alarm unit interrogating the monitored units sequentially by transmitting personalized interrogation signals.
In yet another variant embodiment, the second unit includes a switch for selecting between a mode in which it monitors separation becoming too great and a mode in which it monitors separation becoming too small, and means are provided in the second unit to cause the alarm to operate when, in the mode for monitoring separation becoming too small, the identity information is recognized in a received frame.